1. Field of the Invention
The present invention relates to a method and apparatus for detecting gases and more specifically to an electrochemical cell suitable for detecting numerous toxic gases. The device is suitable for both stationary and portable monitoring.
2. Background Information
The requirement for monitoring toxic gases in the environment has steadily increased in recent years as safety and health professionals have become increasingly aware of the dangers posed by these substances. Greater awareness has further prompted government regulations to address environmental monitoring and related issues. Although such monitoring serves to protect the environment as a whole, the safety of people in the workplace continues to be of most vital concern. In this regard, most toxic gases have various levels or limits, typically set by industry associations or regulatory agencies. Typically, several levels are defined for each type of gas. For example, a threshold limit value (TLV) sets the maximum allowable level of a gas that a person may be exposed to for an eight-hour period, five days a week. The short-term exposure limit (STEL) gives the maximum exposure that a person may be exposed to for a fifteen minute period not to exceed four occurrences per 8-hour work day. The permissible exposure limit (PEL) is the maximum limit a person may be exposed to the gas for any time period. Adherence to these standards requires a toxic gas detector capable of accurate detection of the toxic gases of interest. Further, as these various exposure limits may span a large range of concentrations, the toxic gas detector must accurately measure the concentration over a wide range of concentrations.
Several electrochemical gas sensing devices are now commercially available and used extensively in industry for the primary purpose of safeguarding the work place environment. Unfortunately, these devices are largely based on a common design and therefore share in their shortcomings. Specifically, most available devices are based on fuel cell technology. In general, these devices operate by providing a working electrode and a counter electrode, with an electrolyte disposed between the electrodes. A gas of interest present in the ambient diffuses through a membrane into the electrolyte. The gas itself, or some other chemical species generated by reaction of the gas with the electrolyte, diffuses through the electrolyte. Upon reaching the working electrode, the electroactive species causes a current to flow which can be measured and related to concentration of the gas to be measured.
The fuel cell type devices generally use sulfuric acid based electrolytes, e.g. typically a 1-8 normal (N) solution, which results in the broad spectrum detection of numerous gases with any particular cell. Selectivity is achieved almost entirely on the basis of controlling the potential imposed between the working and counter electrodes. One problem with this method is that gases having half reactions with similar electrode potentials will cause similar readings. Thus, there is little discrimination between co-existing gases having similar redox couples. Therefore, a gas which is of no interest may interfere with detection of a similar gas. A further problem with the use of a sulfuric acid electrolyte is that sulfuric acid solutions in low concentration are subject to evaporation while more concentrated solutions are hygroscopic and readily absorb moisture from the air. Either condition is undesirable as evaporation requires continuous electrolyte maintenance while absorption eventually causes leakage and failure of the device. These conditions may shorten the service life of the device (typically more than a year) and/or may increase the level of service required during the service life of the device.
The prior art devices utilize high surface area, porous diffusion electrodes bonded to gas permeable membranes. In some cases the electrode is made of a powdered electrode material adhered to the membrane by a polymer binder. Alternatively, the electrode may be sprayed, painted, or sputtered onto the membrane. These methods of manufacture result in a highly porous structure. This porous structure is desired in the prior art to create a large surface area for reaction by virtue of the pores into which solution may diffuse.
The fuel type devices are plagued by high residual current flow as the devices react with air from the ambient and water from the electrolyte. This residual current flow contributes significantly to zero drift and consequently, false alarms in the field. Aside from the substantial costs related to false alarms, worker health and safety is threatened as recurring alarms may be disregarded as false. In addition, since the residual current flow is temperature and humidity dependent, ambient changes in temperature and/or humidity further aggravate zero drift, increasing the frequency of false alarms. A significant residual current also decreases substantially the signal-to-noise ratio and consequently, amplifier gain affects the residual current (noise) as well as the signal. Therefore, use of a high amplifier gain to provide for detection at low concentrations may not be practical. Furthermore, zero and span calibration adjustments become interactive and require several back-and-forth adjustments to be properly set.
Since the fuel cell electrodes are micro-porous and extremely high surface area, they typically demonstrate slow rates of response and recovery due to the length of time required for the gas to diffuse to and from the electrode, respectively. The slow response rate limits the early-warning capability of the device as potentially dangerous situations arise. A long response time also increases proportionately the time and materials required to perform accurate calibrations. Excessively high flow rates are often recommended for gas calibration in an attempt to shorten response times. This creates a condition for calibration which differs greatly from the manner in which the sensor normally responds to gas in the environment. With regard to recovery, saturation effects are often observed with even occasional exposure to high concentrations (several hundred ppm) of gas. The saturation effects arise due to the above described time involved for the gas to diffuse away from the electrode. This means unacceptably long periods of recovery ranging from several hours to several days. The electrode composition may also contribute to poor recovery as various gases react chemically and irreversibly with the electrodes.
A further problem with the prior art device is that the signal decays over time, even in the presence of a constant concentration, due to the build-up of above-described diffusion layer. Thus, after the prior art device rises to a given level, the signal will slowly decay over time, even though the gas concentration remains constant.
The prior art devices have an extremely large surface area due to the porous structure, which results in considerable capacitance. Due to capacitive charging, the prior art devices require long start-up periods, from several hours to days, during which time the signal drifts considerably. Such devices require that the electrodes be shorted together or biased with a small voltage during storage to maintain a ready state. Additionally, capacitive currents result in increased noise during operation.
The prior art electrodes are generally comprised of platinum, gold, silver or ordinary carbon (graphite) all of which show aging effects over time. Irreversible sensitivity loss is coincident with electrode aging and limits the service life of the device. In addition, during the serviceable life of the device, such devices are not rechargeable without complete electrode replacement.
The fuel cell type devices generally require a third or reference electrode to maintain a constant potential at the working electrode. Instability at the counter electrode necessitates this use of a separate reference electrode. This complicates the measurement electronics with potentiostat circuitry which further introduces drift and noise in the signal.
What is needed is a method and apparatus for accurately and reproducibly detecting and quantifying concentrations of specific gases, particularly in the low part per million concentration level. What is further needed is a method and apparatus which exhibits minimal zero drift and high signal-to-noise ratio. What is further needed is a method and apparatus which may be started up immediately after storage and which requires no special storage technique such as electrode shorting or biasing. What is further needed is a method and device which requires no electrolyte maintenance for the service life of the device and in which the electrodes are non-degradable so that the device may easily be recharged after normal service life without replacing the electrodes. The method and apparatus should exhibit fast response and recovery and should be essentially unaffected by occasional exposure to high concentrations of gas. The method and apparatus should exhibit high chemical selectivity to allow for monitoring of specific gases. Further, the method and apparatus should allow for a small size device to enable personal, portable monitoring of toxic gas levels.